The present invention relates to generally an exposure control device and more particularly an exposure control device of the type which may positively prevent mechanical shock from causing an erratic exposure due to the erratic rotation from an inoperative position of a step-motor for driving a plurality of shutter blades which also function as the aperture setting blades.
There have been devised and demonstrated various exposure control devices of the type in which a plurality of shutter blades which also function as the aperture setting blades are opened and closed by a step-motor which in turn is driven in response to driving pulse or pulses the number of which is dependent upon the brightness of a subject, whereby the optimum exposure may be made. They are, for example, disclosed in the copending applications filed by the same inventors, Application Ser. Nos. 550,367 filed on Feb. 18th, 1975, 567,231 filed on Apr. 11th, 1975, and 569,006 filed on Apr. 17th, 1975. These exposure control devices have a common object for overcoming the defects encountered in the prior art exposure devices of the type in which the governor or moving coil is used for driving the shutter blades which also function as the aperture setting blades.
In general, in the prior art exposure control devices disclosed in the above copending applications include a first control circuit and a second control circuit. The first control circuit is for controlling the selective energization of a plurality of driving coils or windings of a step-motor so that the latter may rotate through a discrete angle in response to each driving pulse. The second control circuit, which, for instance, includes a shift register, is for controlling the direction of rotation of the step-motor. When the driving coils of the step-motor are energized, the step-motor is rotated through a predetermined angle, and is securely held in position by the magnetic forces produced by the energization of the driving coils. Therefore, the erratic rotation of the step-motor due to the mechanical shock or the like will hardly occur. However, when the driving coils are not energized, the step-motor is held in position only by the magnetic forces of permanent magnets of the rotor or stator. These forces are very weak as compared with the forces produced when the driving coils are energized. As a result, the erratic rotation of the step-motor due to the shock or the like very frequently tends to occur when the driving coils are not energized. Since the step-motor is drivingly coupled to the shutter blades, the mechanical shock or the like imparted to them also tends to cause the erratic rotation of the step-motor, resulting in an erratic exposure. Furthermore, if the step-motor fails to return to its correct initial position (to be referred to as "inoperative position" in this specification), an erratic exposure results.